This invention relates to an electronic circuit for controlling the amount of AC electrical power delivered to an inductive or resistive load.
Many different circuits have been developed to control the AC power delivered to inductive and resistive loads. Most of them use thyristor type semiconductor devices like (SCRs) and (TRIACs) which being conducting when a suitable trigger pulse is applied and continue conducting as long as sufficient current flows through them. Some of them use an analog-type semiconductor device like (MOSFETs) and (DARLINGTON TRANSISTORs) which require a continuous trigger signal to maintain conduction. Both thyristor and transistor type devices have advantages and disadvantages within this context.
The present invention proposes to overcome the aforementioned prior art limitations by including a full range of features such as, for example, dynamically measuring the phase lag in real time and automatically correcting for it, soft start, auxiliary on/off, independently adjustable time delays for the on and off functions, (0-10 volt) and (4-20 ma) input for remote power level control, and a half-wave rectification mode. Thus, the usefulness afforded by this invention is that it provides a circuit that is completely compatible with both thyristor and transistor-type devices. This allows the circuit according to the invention to be used in a wide variety of applications by simply choosing the best switching device for each case. Thus, the circuit according to the invention may be applied to control any load, regardless of the inductive properties of the load, without adjustment or modification.
The inductive property of many loads such as motors, transformers, and coils causes the current in these devices to lag behind the applied voltage. This condition is called phase lag and the magnitude of this lag is a function of the inductance of the load as well as the amount of current flowing through the load. When semiconductor devices are used to control the amount of power delivered to the load, the timing of the trigger signals must be adjusted to compensate for the phase lag. Several circuits exist, a current transformer-type system or a load sensing resistor type system, which make these kinds of adjustments with suitable modifications, but none of these circuits can automatically correct for phase lag for any load, regardless of the inductive properties of the load.